The present invention relates to a martensitic stainless steel suitable for use in oil wells and gas wells (hereinafter collectively referred to as "oil wells"). More particularly, the invention pertains to a martensitic stainless steel for use in oil wells having excellent corrosion resistance sufficient to withstand severe corrosive environments which contain corrosive impurities such as carbon dioxide, hydrogen sulfide, and chloride ions while retaining a proper level of strength. The martensitic stainless steel is also useful in linepipe.
Recently, the environments in wells for collecting oil and natural gas have become increasingly severe. Not only are oil wells drilled deeper, but also the number of wells which contain carbon dioxide and/or hydrogen sulfide has increased. As a result, embrittlement of the materials used in oil wells due to corrosion or similar damage has become a serious problem.
Carbon steels and low alloy steels were used for many years to manufacture oil well tubing, which is one of the members generally used in oil wells. However, as the environments in oil wells become severer, the proportion of alloying elements added to steels for use in oil wells has been increased. For example, addition of Cr is known to improve the corrosion resistance of a steel in oil wells which contain carbon dioxide in a relatively high concentration, and Cr-containing steels such as 9%Cr-l%Mo steel and JIS SUS 420 martensitic stainless steel, which contain 9% and 13%, respectively, by weight of Cr, are frequently used in such oil wells.
However, such steels which contain a relatively large amount of Cr do not have good corrosion resistance to hydrogen sulfide. Therefore, the use of these steels in environments which contain hydrogen sulfide along with carbon dioxide is substantially limited since they are susceptible to sulfide stress corrosion cracking in such environments.
In such severe corrosive oil well environments containing both carbon dioxide and hydrogen sulfide, it is necessary at present to use a duplex stainless steel or austenitic stainless steel having a further high content of alloying elements. However, such a steel having an increased content of alloying elements significantly adds to the material costs.
Various attempts have been made to develop a relatively inexpensive steel for use in oil wells which is based on the above-described JIS SUS 420 martensitic stainless steel and which has improved corrosion resistance so as to withstand H.sub.2 S-containing severe corrosive environments.
Japanese Patent Publication No. 3-2227(1991) discloses a JIS SUS 420-based low-C steel having improved resistance to stress corrosion cracking in H.sub.2 S-containing environments. The steel contains 3.5-6% Ni and 0.5-3% Mo and has a decreased carbon content of 0.02% or less on a weight basis.
Japanese Unexamined Patent Applications Laid-Open Nos. 2-243740(1990) and 3-120337(1991) each disclose a steel JIS SUS 420 having good resistance to sulfide stress corrosion cracking, characterized by decreased Ni and Mo contents, and addition of one or more of Ti, Nb, V, and Zr or decreased Mn and S contents.
Japanese Unexamined Patent Applications Laid-Open Nos. 61-106747(1986) and 62-54063(1987) disclose a low-C, Ca-containing martensitic stainless steel in which Zr and Ti may be added.
Japanese Patent Publication No. 3-60904(1991) describes a martensitic stainless steel for seamless tubes which may contain one or more of various alloying elements including Ni, Mo, Cu, Nb, V, Ti, and Ca and which have limited S and P contents.
Japanese Unexamined Patent Application Laid-Open No. 61-207550(1986) discloses a boron-containing martensitic stainless steel suitable for use in acidic oil wells.
Japanese Unexamined Patent Application Laid-Open No. 2-243739(1990) discloses a martensitic stainless steel for use in oil wells which contains 15%-19% by weight of Cr.
It is known that a decrease in hardness or softening of a martensitic steel accompanied by a decrease in tensile strength is effective for reducing its susceptibility to sulfide stress corrosion cracking. Nickel-containing martensitic stainless steels as described in Japanese Patent Publication No. 3-2227 (1991) have an Ac.sub.1 point which is much lower than that of the conventional JIS SUS 420 steel. Accordingly, there is a need for a Ni-containing martensitic stainless steel which can be readily softened by tempering at a low temperature which is below the decreased Ac.sub.1 point.
The incorporation of additional alloying elements such as Ti, Nb, V, and/or Zr in a martensitic stainless steel as taught by Japanese Unexamined Patent Applications Laid-Open Nos. 2-43740(1990) and 3-120337(1991) serves to stabilize strength after tempering. However, upon further investigations, it was found that the resulting steel sometimes shows an abnormal fluctuation in strength, which is a problem in production of the steel on a commercial scale.